//练习6.1: 为bit数组实现下面这些方法
//
//func (*IntSet) Len() int      // return the number of elements
//func (*IntSet) Remove(x int)  // remove x from the set
//func (*IntSet) Clear()        // remove all elements from the set
//func (*IntSet) Copy() *IntSet // return a copy of the set
package main
